Physics · 12th Grade · Energy and Momentum Systems · Weeks 10-18

Work and Power

Students will define work and power, calculating them in various physical scenarios.

• Common Core State Standards
• HS-PS3-1

About This Topic

Work and Energy Conservation introduces the concept of energy as a conserved quantity that changes form but is never lost. Students analyze how work, the transfer of energy via force, transforms potential energy into kinetic energy or thermal energy. This topic is central to HS-PS3-1, where students create computational models to track energy changes within a system.

This unit connects to real-world issues like renewable energy and mechanical efficiency. By calculating the work done by non-conservative forces like friction, students understand why no machine is 100% efficient. This framework allows them to solve complex problems that would be difficult using Newton's Laws alone, such as the motion of a roller coaster on a curved track.

Students grasp this concept faster through structured discussion and peer explanation of energy 'accounting' in closed systems.

Key Questions

1. Differentiate between the scientific definition of work and its everyday usage.
2. Analyze how the angle between force and displacement affects the work done on an object.
3. Evaluate the power output of a machine given the work it performs over a specific time.

Learning Objectives

• Calculate the work done on an object when a constant force is applied over a specific displacement.
• Analyze how the angle between the applied force and the displacement vector affects the amount of work performed.
• Evaluate the power output of a system or machine given the work done and the time taken to perform it.
• Differentiate between the physics definition of work and its common, everyday meaning.

Before You Start

Vectors and Scalars

Why: Students need to understand the difference between scalar quantities (like distance) and vector quantities (like displacement and force) to correctly apply work and power formulas.

Newton's Laws of Motion

Why: Understanding force as a push or pull and its effect on motion is fundamental to defining and calculating work.

Key Vocabulary

Work (Physics)	Work is done when a force causes an object to move a certain distance. It is calculated as the product of the force component in the direction of motion and the displacement.
Power (Physics)	Power is the rate at which work is done or energy is transferred. It is calculated by dividing the work done by the time it takes to do that work.
Displacement	Displacement is a vector quantity representing the change in an object's position from its starting point to its ending point, including direction.
Force	A force is a push or pull upon an object resulting from the object's interaction with another object.

Watch Out for These Misconceptions

Common Misconception

Energy is 'used up' or disappears.

What to Teach Instead

Energy is only transferred or transformed. Using thermal cameras to look at a track after a car passes helps students see that 'lost' energy has actually turned into heat.

Common Misconception

Work is done whenever a force is applied.

What to Teach Instead

Work only occurs if the force causes a displacement in the direction of the force. Having students push against a wall versus lifting a book helps clarify that effort does not always equal physical work.

Active Learning Ideas

Inquiry Circle: Roller Coaster Design

Groups design a marble track with loops and hills. They must calculate the minimum starting height required to complete the loop, accounting for energy lost to friction and sound.

90 min·Small Groups

Gallery Walk: Energy Transformation Stories

Stations show images of different systems (a solar panel, a person jumping, a toaster). Students move in groups to write the 'energy story' for each, identifying every transformation from start to finish.

40 min·Small Groups

Think-Pair-Share: The Bouncing Ball Mystery

Students observe a ball that doesn't bounce back to its original height. They discuss in pairs where the 'missing' energy went and how to prove it still exists in the system.

15 min·Pairs

Real-World Connections

• Engineers designing lifting mechanisms, such as cranes or elevators, calculate the work done against gravity and the power required to move heavy loads efficiently.
• Athletic trainers analyze the power output of athletes during activities like sprinting or weightlifting to assess performance and design training programs.
• Mechanics determine the power of car engines by measuring the work they perform to accelerate the vehicle over a given distance and time.

Assessment Ideas

Quick Check

Present students with three scenarios: 1) Pushing a box across a floor, 2) Holding a heavy box stationary, 3) Carrying a box up stairs. Ask students to identify which scenario involves scientific work and explain why, using the terms force and displacement.

Exit Ticket

Provide students with a problem: A 50 N force pushes a box 10 m horizontally. Calculate the work done. Then, ask them to calculate the power if this took 5 seconds. Include a question asking them to explain the difference between this calculation and the everyday meaning of 'working hard'.

Discussion Prompt

Pose the question: 'If you push a wall with all your might, but the wall doesn't move, how much work have you done according to physics? How does this differ from how you might describe your effort?' Facilitate a class discussion comparing scientific work and everyday effort.

Frequently Asked Questions

What is the work-energy theorem?

It states that the net work done on an object is equal to its change in kinetic energy. It is a powerful shortcut for finding the final speed of an object without needing to calculate acceleration over time.

How do we define a 'system' in energy problems?

A system is whatever collection of objects you choose to study. If you include the Earth in your system, you talk about gravitational potential energy. If you don't, you talk about the work done by gravity.

How can active learning help students understand energy conservation?

Active learning strategies like 'Energy Bar Charts' (LOL diagrams) allow students to visually track energy flow. By physically drawing the 'storage' of energy at different points in a simulation, students move away from rote formula memorization and toward a conceptual understanding of energy as a substance-like quantity that moves between accounts.

Why is power different from energy?

Energy is the total amount of work done, while power is the rate at which it is done. A lightbulb and a lightning bolt might involve the same energy, but the lightning bolt has much higher power because it releases it instantly.